1. Field of the Invention
The present invention relates to a nitrogenoxygen separating molecular sieve and separator, more specifically, a molecular sieve comprised of carbon fibers capable of separating and purifying a high purity of nitrogen gas from air and a separator using such a sieve.
2. Description of the Related Art
A large quantity of nitrogen gas is used in industry for creating inert atmospheres in heating furnaces, etc. It is known to prepare nitrogen gas by liquefying air and fractionally distilling nitrogen gas from the liquid air using the difference of the liquefying temperatures. Nitrogen gas prepared by liquefaction and fractional distillation of air is, however, relatively expensive.
A nitrogen gas generating apparatus using an inorganic material of the zeolite series is also known, but a drying device is necessary in the apparatus at a stage before an adsorbing column due to the high water adsorption of zeolite, making the cost of the apparatus high.
Nitrogen gas generators using molecular sieves are also commercially available. Nitrogen gas generation with molecular sieves enables preparation of nitrogen gas in large quantities and a very low cost in relation to nitrogen gas generation by liquefaction and fractional distillation of air.
A typical molecular sieve material used in commercially available nitrogen gas generators is "molecular sieve carbon", a particulate carbon material. Various processes have been disclosed for producing such molecular sieve particulate carbon. In one process, raw materials of a phenol or furan series resin are adsorbed in the surfaces of a porous carbon adsorbent and then polymerized and/or condensed. Carbonization is then carried out to form a fine porous structure in the adsorbent (Japanese Examined Patent Publication (Kokoku) No. 49-37036). In another process, a hydrocarbon which will produce carbon by thermal decomposition is added to coke, which is then heat-treated to deposit carbon into the fine pores of the coke (Japanese Examined Patent Publication (Kokoku) No. 52-18675). In still another process, an organic material which is tacky at room temperatures is mixed with fine particulate coal char, which is then granulated and carbonized (Japanese Unexamined Patent Publication (Kokai) No. 57-175715).
The pore sizes of fine pores of a nitrogenoxygen separating molecular sieve, however, must be controlled to within the range of 0.5 nm or less, preferably from approximately 0.35 nm to approximately 0.5 nm. All methods for producing a nitrogen-oxygen separating molecular sieve from particulate carbon involve very complicated and highly controlled procedures.
Due to the inherent features of raw materials of porous particulate carbons, molecular sieve particulate carbons have the construction of fine pores as shown in FIG. 1, which includes macropores (with a large pore size) 1, transitional pores (with intermediate pore sizes) 2, and micropores (with finest pore sizes) 3, continuously from the surface into the interior of a particulate carbon. Thus, effective micropores 3 having a pore size of from 0.35 nm to 0.5 nm are formed only in the interior of the particles. Therefore, a relatively high adsorbing pressure and deadsorbing vacuum are necessary for pressure swing adsorption in a nitrogen generator. The high adsorbing pressure and deadsorbing vacuum necessitate expensive vacuum pumps and other devices. Further, a high adsorbing pressure and/or deadsorbing vacuum can split or powder molecular sieve particulate carbons due to the mechanically weak pore structure of the molecular sieve particulate carbons
Further, molecular sieve particulate carbons have a relatively small effective geometrical surface area, increasing the size of a nitrogen generator.
A process for producing a molecular sieve in the form of fiber, i.e., a molecular sieve carbon fiber, has also been disclosed (Japanese Unexamined Patent Publication (Kokai) No. 57 -101024). This process includes melt spinning a material depolymerized from coal, produced by a special method, followed by infusibilization and slight carbonization and activation. This process, however, requires a special raw material for spinning and, more importantly, cannot produce carbon fibers having a narrow distribution of a pore size in the range of 0.5 nm or less with a large effective fine pore volume. It activates the carbon fibers from the outside with steam, etc. Steam, etc. at a low temperature of 650.degree. C. to 700.degree. C. is not effective for activation. Therefore, even if effective fine pores having a pore size of 0.35 to 0.5 nm may be formed, they are few in number. Activation at a higher temperature to increase the volume of pores makes those pore sizes too large, i.e., larger than 0.5 nm. Thus, control of activation is extremely difficult or impossible. In any case, activation of fibers from the outside is not sufficient for providing molecular sieve carbon fibers effective for separating nitrogen and oxygen. The examples or other portions of the above patent publication, therefore, describe or mention only separation between benzene and cyclohexane, not between nitrogen and oxygen.